The dominant allele H reduces the number of body bristles in fruit flies, giving rise to a hairless phenotype. In the homozygous condition, H is lethal. The dominant allele S has no effect on bristle number except in the presence of H, in which case a single S allele suppresses the hairless phenotype, thus restoring the bristles. However, S is also lethal in homozygotes. a. What ratio of flies with normal bristles to hairless individuals would we find in the live progeny of a cross between two normal flies both carrying the H allele in the suppressed condition? b. When the hairless progeny of the previous cross are crossed with one of the parental normal flies from part (a) (meaning a fly that carries H in the suppressed condition), what phenotypic ratio would you expect to find among their live progeny?

a. If H allele is present, the fly will be hairless. But H allele is lethal at homozygous condition, so all hairless flies will be heterozygous for H locus (Hh). S allele has no effect on bristle number, but in presence of H allele, S produces a "normal" phenotype and is also lethal at the homozygous condition. So genotypes and phenotypes will be like this:

- SSHH: No viable- Lethal

- SSHh: No viable-Lethal

- SShh: No viable-Lehtal

- SsHH: No viable-Lethal

- SsHh: Normal

- Sshh: Hairless

- ssHH: Normal

- ssHh: Normal

- sshh: Normal

A cross between two normal flies carrying the two alleles (SsHh) will produce (according to the attached Punnett Square):

6 Normal (SsHh, ssHh, and sshh)

2 Hairless (Sshh)

Therefore, the ratio of flies with normal bristles to hairless individuals will be 3:1.

b. When the hairless progeny of the previous cross (Sshh) are crossed with one of the parental normal flies from part a (SsHh), it will be produced (according to the second Punnett Square):

8 Normal (SsHh, ssHh, and sshh)

4 Hairless (Sshh)

So, the ratio of flies with normal bristles to hairless individuals will be 2:1.